An electron microscopical observation was carried out to compare the general shape of the mast cells and structures of granules inside the cells in the stomach of 5 species in 3 orders of Mammals. In convenience, the granules in the cytoplasm were abbreviated as follows: 1) Homogeneous granule, GR1 2) Particulate granule, GR2 a. Dark dense particulate granule, GR2-1 b. Less dense particulate granule, GR2-2 3) Reticular granule, GR5 a. Dark dense reticular granule, GR5-1 b. Light dense reticular granule, GR5-2 In Mammalia including goat, dog, cat, and hamster, most of cytoplasmic organelle were Golgi apparatus and mitochondria, and most of the cytoplasmic granules were highly densed GR1and GR2. However GR5-1 and GR5-2 appeared in guinea pig while one side sunken or crescent-like types occured in both dog and guinea pig. All mast cells were oval or spindle with cytoplasmic processes around the cell. There was also found vacuoles and vesicles in these cells. These results demonstrated that there was a morphological difference between species of vertebrate in the mast cells and their cytoplasmic granules. It was also suggested that a variety of structures of granules were closely related with the composition (histamine, heparin, serotonin, hyaluronic acid etc.) and mature of the granules.